Chemistry Reference
In-Depth Information
Most of the elements on the Periodic Table of Elements exist as solids
at standard temperature and pressure. Much of the matter that you en-
counter on a daily basis is also found in the solid phase. Probably the two
most interesting things about solids are that they are mostly made up of
empty space and that even their molecules are in constant motion.
Solids aren't really solid? They are mostly empty space? When we dis-
cuss the structure of an atom in Chapter 3 you will learn that the size of the
nucleus is actually very tiny compared to the size of the atom. The space
that an atom effectively occupies really has to do with the fact that the
electrons are moving very quickly and that the repulsive forces between
electrons of different atoms keep them a certain distance apart. If you could
strip the electrons off an atom and neutralize the charge of the nucleus,
the atom would occupy only a fraction of its original size. To picture the
size change, if the original size of the atom was about the size of a baseball
stadium, the altered atom would be about the size of a marble. Just think:
The “solid” floor that you stand on is almost completely empty space!
Did you know that the molecules of a solid are in constant motion? We
tend to think of solids as static, but the truth is that they are made up of
particles that are moving. In a solid, the motion of these molecules is re-
stricted to mainly vibrational motion, meaning that each particle stays in
the same position relative to each other, but they are still vibrating in place.
What we call “temperature” is a measure of the average kinetic energy of
the particles of a substance, so if the temperature of an object is above zero
on the Kelvin scale, neither the mass, nor the average velocity of its par-
ticles, could be zero.
The limitations on the motion of the particles of solids account for the
fact that they are said to have definite shape and definite volume. By defi-
nite shape, we mean that they do not take the shape of a container, the way
that gases and liquids will. By definite volume, we mean that they do not
expand or contract to occupy the entire container. This does not mean,
however, that the volume of a solid is a constant. Solids do expand and
contract in response to temperature changes. That is why, when you look
up the density of a solid, it will always indicate the temperature at which
the value for density is listed.
Another interesting thing about solids is that all true solids have what
is called crystalline structure. By this, we mean that their particles are ar-
ranged in a three-dimensional, orderly pattern. Some things that most
people think of as true solids, such as glass, are not considered true solids
to chemists, because they don't possess this crystalline structure.
